This is a competing continuation proposal requesting support of a research program involving the elucidation of the properties and nature of excited molecular oxygen in the context of the biological cell and several model systems. The specific aims are: (i) Investigations into the dynamic properties of excited molecular oxygen in heterogeneous fluid systems suchas aqueous micelles, reverse micelles, vesicles, liposomes and ghost cells. (ii) Studies of carotenoid and related polyene reactivity towards singlet oxygen especially in media of different polarity. (iii) Measurements of the relationship between type I and type II photodynamic mechanisms when light absorbing sensitizers are associated with biopolymeric materils, e.g., DNA and proteins. (iv) Detailed investigations of the single electron transfer to excited molecular oxygen from biological reductants, including a study of medium effects. These studies are prompted by the overall need to know much more detail about how excited molecular oxygen, behaves in the heterogeneous environments of living systems. The experimental methods of photokinetics will be employed including laser flash photolysis coupled to kinetic spectroscopy with namosecond time resolution. It has been clearly demonstrated during our extant program that such methods are effective in the pursuance of these objectives.